(1) Field of the Invention
The present invention relates to a toner for electrophotography. More particularly, the present invention relates to a blue toner which is excellent in the negative chargeability and the sharpness of a formed image.
(2) Description of the Prior Art
A toner formed by dispersing a black pigment such as carbon black in a binder resin medium is ordinarily used for electrophotography. However, with a recently increased demand for color copies, toners formed by dispersing various chromatic coloring agents in binder resin media are gradually being used in the art.
Of these color toners, toners comprising a phthalocyanine pigment such as copper phthalocyanine are mainly used as blue toners from the viewpoint of color sharpness. However, these phthalocyanine pigments tend to be positively charged and hence, they are defective in that they cannot be used for photosensitive plates bearing an electrostatic latent image of positive polarity, such as a selenium photosensitive plate. Of course, it may be considered that this disadvantage will be obviated if a charge controlling agent capable of imparting a negative chargeability to copper phthalocyanine is incorporated. However, incorporation of a large quantity of this charge controlling agent renders the toner per se moisture-sensitive or causes reduction of the electric characteristics of the toner.
Furthermore, there has been proposed and adopted a method in which a phthalocyanine pigment is rendered negatively chargeable by substituting the benzene ring of the basic skeleton of the phthalocyanine with a halogen such as chlorine.
However, this substitution with a halogen often changes the color from blue to green, though negative chargeability can be given by this substitution.
As another method, there is known a method in which a sulfonyl group is introduced into the phthalocyanine ring to effect conversion to a lake. However, in this method, the negative chargeability by friction is insufficient.